The present invention relates to a camera which is operable in two operative modes: a normal photographing mode using a photographing film; and a digital photographing mode using an imaging device such as a CCD (Charge Coupled Device) instead of the photographing film.
Recently, imaging devices, such as CCDs, have improved greatly. In particular, the density of image pixels, i.e., the number of pixels per unit "area", has been increased such that the quality of captured images has improved greatly. Digital cameras using such imaging devices have been developed.
On the other hand, in normal photography, a new film system called "APS" (Advanced Photographing System) has been developed which makes normal photographing easier.
In order to make use of the advantages of both the normal photographing and the digital photographing systems, a camera operable in the normal photographing mode and the digital photographing mode has been developed. An example of such a camera is provided with a half mirror on an optical axis of a photographing lens. The half mirror splits an incident light into two light rays: one is directed towards the photographing film; the other is directed to the imaging device.
In such a camera, since the light converged by a lens system is split into two rays, the amount of light incident to the photographing film or the imaging device is reduced to one-half in comparison to a camera which is operable only in only one mode (digital or normal). Since recent cameras are designed to have a small diameter photographing lens, this reduction in the amount of light incident to the film or the imaging device becomes a problem.
One method of avoiding the above problem is to have a light path switching mechanism. For example, a total reflecting mirror may be inserted in or retracted from the light path, such that the path is switched, so that the light is incident on either the photographing film or the imaging device.
However, such a light path switching mechanism will require space, and as such, a camera including such a mechanism must be relatively large to have sufficient room for accommodating the mechanism. Further, the structure may be very complicated and increase manufacturing cost.
Japan Patent Provisional Publication HEI 2-101874 discloses a camera which is switchable between the normal photographing mode or the digital photographing mode with a relatively simple structure. In particular, a pressure plate at the rear of the camera is provided with a transparent window member, and an imaging device is arranged behind the transparent window member. In this camera, when the photographing film is loaded in the camera, the normal photographing is executed, while when the photographing film is not loaded in the camera, the digital photographing can be executed. That is, when the film is not loaded, the image focused by a photographing lens is transmitted to the imaging device through the transparent window member. Note that according to this system, the light amount is the same for both the normal and digital photographing.
In the camera described above, the structure requires that a film surface and an image receiving surface of the imaging device are at different distances from the lens with the transparent member located therebetween. In order to obtain an in-focus image in either mode, the image must be focused at different positions depending on the mode. In other words, a focusing operation must be performed that compensates for the difference between the image receiving planes, i.e., the postions of the two film and the image receiving surface of the imaging device. The above publication discloses a structure for automatically switching focusing operations in accordance with a selected operation mode. In order to perform focusing operations with respect to both of the different positions, a detector for detecting a focusing status of one of the positions and/or a logic circuit for performing a focus calculation is used for each focusing position, which duplicates the number of focus detecting systems or complicates the logic of the logic circuit, and as a result a price of the camera is increased.